1. Field of the Invention
The present invention relates to an illumination device for use in a mobile phone, a portable TV set, a hand-held game machine, a personal digital assistant (PDA), and a notebook personal computer, and to a display device using the same.
2. Description of the Related Art
Known examples of a conventional illumination device which is called a backlight or a frontlight include a side light type illumination device in which a plurality of LED light sources are disposed on a side of a light guide plate as the light source. For example, an illumination device for use in a portable information device such as a mobile phone is required to be thinner. However, since it is difficult to make thinner an LED as the light source, it is difficult to realize a thinner illumination device or a thinner display device. One approach to solving the problem is disclosed in JP 2000-249837 A where a wedge-like light guiding portion couples an ordinary-sized LED and a thin light guide plate.
In an illumination device having a conventional wedge-like light guiding portion, the angle of incidence of propagating light with respect to a reflecting surface becomes greater as the light is repeatedly reflected by the inner surfaces of the light guiding portion, so the amount of light propagating through the light guide plate with the angle of incidence being greater than the critical angle decreases, and thus, light cannot be efficiently guided from the light source to the light guide plate.
Further, a greater angle of incidence of propagating light results in a greater amount of leakage light to the outside from the light guiding portion and adjacent areas. Therefore, unevenness in brightness and stray light increase, and satisfactory illumination cannot be carried out.
Still further, the number of reflections of light inside the light guide plate is large. Thus, if a group of conventional parallel sawtooth prisms are formed on the light guide plate, it is difficult to form an even distribution of emitted light.